PDPs belong to a broad classification of displays known as vacuum fluorescent displays. The mechanism underlying their operation is quite similar to that operating in fluorescent lighting. That is, an electrical discharge is initiated in a gas mixture (Penning gas) at low pressure, this discharge ionizing the gas atoms releasing ultraviolet radiation which strikes a phosphor emitting visible light. Where color is desired, phosphors which emit red, blue, and green light upon being struck by the ultraviolet energy are used.
Plasma displays can be classified broadly into one of three types, depending upon the type of voltage utilized to address the display. Thus, there are DC displays, AC displays, and hybrid displays, the last type comprising a AC-DC mode of display. Each type exhibits its own advantages and disadvantages; for example, the ease of manufacture (DC type) or improved efficiency (AC type). The principal substantive difference between those displays resides in the method by which the current is limited to prevent arcing of the Penning gas. In the DC panel this is accomplished through the incorporation of an external resistor in series with the anode. In the AC and hybrid display panels a thin dielectric glass layer is applied as a coating on both electrodes.
A typical PDP panel is composed of two flat glass sheets, customarily having a thickness of about 2-3 mm and spaced about 100-150 .mu.m apart. Barrier ribs, commonly consisting of a glass frit, define this gap as well as the individual subpixels. The electrodes are located on the interior surface of these glass sheets. In the simplest case the electrodes are orthogonally directed between the top and bottom substrate glass. Typical electrode materials include Cu/Cr and Ag which are applied as a paste frit.
Covering the electrodes in either the AC or AC-DC hybrid type of display panel is an appropriately doped dielectric glass frit (which is highly reflective when applied on the back surface and highly transparent layer when applied on the front) which, in turn, is covered with a sputtered MgO layer. This latter layer serves both as a protector of the underlying layers and a low energy source of electrons which assist in sustaining the plasma. A final layer is put in place which comprises an appropriate fluorescent material applied over the MgO.
Soda lime glass sheet formed by means of the float process has, to date, been the principal choice as the substrate material, largely on the basis of its availability. As the resolution demands of the display have increased, however, PDP manufacturers are finding this glass to be less than optimal. For example, going from a monochrome display to color requires greater than a fourfold increase in resolution as four subpixels, one blue, one red, and two green comprise the same area as one monochrome pixel. Changes in the dimensions and flatness of the glass during the multiple firings required in the display fabrication lead to variations in cell gap width which, in turn, may lead to variations in the electrical characteristics of the cell, resulting in color variations or, in the extreme case, to dead pixels.
Laboratory work and manufacturing experience have indicated that a good initial measure of the thermal stability of a glass is provided by its strain point. The typical soda lime glass exhibits a strain point in the vicinity of 530.degree. C. The multiple firings discussed above are carried out at temperatures in the range of 500.degree.-585.degree. C.; that is, temperatures which can be above the strain point of the soda lime glass. It has been estimated that, to maintain sufficient stability during those firings, the strain point of the substrate must necessarily be higher than 600.degree. C.
A second critical property requirement of the glass substrate is an appropriate coefficient of thermal expansion. The coefficient exhibited by the glass must be compatible with those of the low firing temperature frits utilized for the edge seal, the rib structure, and the dielectric layers (in the AC and AC-DC hybrid displays). Most of the currently employed frits have been developed to be compatible with soda lime glass whose linear coefficient of thermal expansion over the temperature range of 0.degree.-300.degree. C. is approximately 85.times.10.sup.-7 /.degree.C. One example of such a frit commercially used in color PDPs is Corning Code 7599 frit marketed by Corning Incorporated, Corning, N.Y., which has a linear coefficient of thermal expansion (0.degree.-300.degree. C.) of 89.times.10.sup.-7 /.degree.C. and is conventionally fired at temperatures of about 580.degree.-585.degree. C. Generally, the coefficient of expansion and the firing temperature of a frit are inversely related; that is, the lower the coefficient of thermal expansion, the higher the required firing temperature. Accordingly, to maintain a sufficiently low firing temperature, it is desirable to utilize a substrate glass demonstrating a linear coefficient of thermal expansion (0.degree.-300.degree. C.) greater than 70.times.10.sup.-7 /.degree.C., but less than 90.times.10.sup.-7 /.degree.C., and, preferably in the range of 79-85.times.10.sup.-7 /.degree.C.
A third vital property which the glass substrate must possess is a compatibility with a sheet forming process for glasses. The most widely used sheet forming processes include rolling, slot drawing, float, and overflow downdraw. Float is currently the process of choice to fabricate sheet operable in PDPs because of its inherent economy in large scale production and its ability to provide a sufficiently flat and smooth substrate. Three critical properties required in glass compositions to render them operable in the float process have recently been enumerated in the following three European Patent Applications: 510,543, 559,389, and 576,362. Those properties are: (1) a sufficiently low melting temperature to assure good melting performance and glass homogeneity; (2) a 1000 MPas (10,000 poises) isokom temperature, which temperature corresponds to delivery of the molten glass onto the float bath, of less than 1240.degree. C.; and (3) a viscosity at the liquidus temperature greater than 300 MPas (3000 poises).
Accordingly, the primary objective of the present invention was to develop glass compositions exhibiting strain points higher than 600.degree. C., linear coefficients of thermal expansion over the temperature range of 0.degree.-300.degree. C. between 70-90.times.10.sup.-7 /.degree.C., 1000 MPas isokom temperatures below 1240.degree. C., and viscosities at the liquidus temperature greater than 300 MPas.
A second objective of the subject invention was to devise glass compositions especially suitable for use as substrates in plasma display panels.